1. Field of the Invention
The present invention relates to an SSB modulation and apparatus, and more particularly to an SSB modulation and demodulation apparatus having a variable communication bandwidth.
2. Related Background Art
An SSB modulation/demodulation apparatus modulates an audio signal into an SSB modulated wave having a single side band component, and demodulates an SSB modulated wave into an original audio signal. Such an SSB modulation/demodulation apparatus is used in an SSB transmitter/receiver capable of performing communication with a low power and narrow communication bandwidth.
FIG. 6 is a block diagram showing a conventional analog SSB modulation apparatus.
An analog modulating wave D.sub.A of audio signal is inputted to a band-pass filter (BPF) 10 to restrict the bandwidth of the analog modulating wave. The analog modulating wave from BPF 10 is inputted to first and second phase shifters 12 and 14 to Hilbert-convert it into first and second modulating waves each having a phase difference of 90 degrees, which are then supplied to first and second multipliers 16 and 18, respectively.
The first and second phase shifters 12 and 14 constitute a Hilbert converter.
The first and second multipliers 16 and 18 are inputted with first and second carriers, respectively. The first and second carriers are generated by a carrier generator 20, and each has a carrier frequency fc and a phase difference of 90 degrees. The first and second modulating waves are respectively multiplied by the first and second carriers to obtain first and second multiplication signals.
The first and second multiplication signals are added together by an adder 22 to generate an analog SSB (LSB) modulated wave. If a subtracter is used in place of the adder 22, an analog SSB (USB) modulated wave is generated.
For example, if an audio signal is intended to be transmitted while retaining a good quality of sounds, the bandwidth of BPF 10 is made broad, whereas if a number of stations are intended to be communicable at the same time, the bandwidth is made narrow. In such a case, two BPFs 10-1 and 10-2 one with a broad bandwidth and the other with a narrow bandwidth are used as shown in FIG. 7 wherein upon a user designation of a desired bandwidth, a bandwidth changeover instruction is generated to activate switches 24 and 26.
The first and second phase shifters 12 and 14 used in analog Hilbert conversion are constructed of a pair of networks having a pass band over all frequency ranges. With a fixed order of each phase shifter, if the bandwidth for ensuring the 90 degree phase difference between the first and second phase shifters 12 and 14 is narrow, the precision of Hilbert conversion characteristics may be made high so that the sideband suppression is made high (refer to a solid line curve A shown in FIG. 2). On the other hand, if the bandwidth for ensuring the 90 degree phase difference is broad, it is necessary to lower the precision of Hilbert conversion characteristics, so the sideband suppression is degraded (refer to a broken line curve B shown in FIG. 2).
For this reason, the Hilbert conversion characteristics of the first and second phase shifters 12 and 14 are arranged to be changed upon changeover of the bandwidth of BPF. If a broad bandwidth of BPF is selected, the bandwidths of the first and second phase shifters 12 and 14 are also made broad, while being contended with a low sideband suppression. On the contrary if a narrow bandwidth of BPF is selected, it is desirable that the bandwidths of the first and second phase shifters 12 and 14 are also made narrow to obtain a high sideband suppression.
However, the Hilbert converter of the first and second digital phase shifters 12 and 14 constructed of analog circuits has essentially unstable Hilbert conversion characteristics, and moreover the Hilbert conversion characteristics have a critical change during its adjustment and are difficult to be adjusted From this reason, the bandwidth of a conventional Hilbert converter has been remained unchanged even if the bandwidth of BPF is changed.
The frequency versus gain characteristics of a conventional SSB modulation apparatus are shown in FIG. 8, with respect to the pass band (for lower sideband) and stop band (for upper sideband).
The solid line curve A shown in FIG. 8 stands for a broad bandwidth of BPF, and the broken line curve B for narrow bandwidth. For both the narrow and broad bandwidths, the sideband suppression is low and spurious signals of certain level are generated.
If the bandwidth of a Hilbert converter were changed in correspondence with a change in the bandwidth of BPF, the frequency versus gain characteristics will become, for example, as shown in FIG. 9.
As shown by the solid line curve A in FIG. 9, spurious signals of high level may be generated because of variation in the Hilbert conversion characteristics relative to a given bandwidth of BPF.
Similar problems of sideband suppression is also associated with an SSB demodulation apparatus which uses a Hilbert converter constructed of analog circuits Namely, the Hilbert conversion characteristics are remained broad, so that the sideband suppression is poor even if the narrow bandwidth of BPF is selected, and interference of certain level is inevitable.
Further, if the bandwidth of a Hilbert converter is arranged to be changed, there is a possibility of high level interference because of variation in the Hilbert conversion characteristics.